Wavelength Division Multiplexing (WDM) techniques in optical fiber systems have been utilized to significantly enhance the data carrying capacity of optical fibers. Essentially, in a WDM system multiple information streams are simultaneously transmitted on a single optical fiber at different wavelengths or channels. Early WDM systems transmitted up to four distinct channels over a single fiber. Recent technological advances are, however, allowing ever-increasing numbers of channels to be transmitted over a single fiber. Generally, systems that transmit in excess of four channels are referred to as Dense Wavelength Division Multiplexed (DWDM) systems in recognition of the closer spacing between the respective channels.
In a typical optical WDM or DWDM communication system, the distinct optical wavelengths or channels are multiplexed and propagated over an optical medium to a plurality of receivers. To ensure interoperability with other system equipment, the channels or wavelengths chosen for transmission, as well as the channel spacings, are selected to correspond to an International Telecommunication Union (ITU) channel grid. According to one such ITU channel grid, the channel spacing is 100 GHz with, for example, channel 15 at 191,500.00 GHz and channel 72 at 197,200.00 GHz.
One or more of the propagated ITU channels are selected for detection within the receiver by interposing appropriate filters between the medium and each receiver. For example, optical signals from each of N different optical signal generators with ITU channel wavelengths of .lambda..sub.1, .lambda..sub.2, . . . , .lambda..sub.N, respectively, are multiplexed and propagated over a system fiber connecting the various receivers. A given filter may pass only one of the ITU channel wavelengths, e.g., .lambda..sub.i, from the multiplexed wavelengths present on the fiber through to the associated receiver, while the other wavelengths are reflected.
Obviously, the ability of the filter to effectively pass the desired channel(s) or wavelength(s) is critical to the operation of the overall system. Another important aspect of the filter is its effect on the system loss budget, i.e., the total amount of optical loss that a given optical link can tolerate while maintaining signal integrity. One type of filter which has been successfully employed in Wide Area Networks (WANs) is a diffraction grating. Diffraction gratings generally offer appropriate spectral resolution for reliably passing a plurality of selected channels. Unfortunately, however, diffraction gratings are bulky, lossy, and expensive. The expense of diffraction gratings and their effect on system loss budget, makes diffraction gratings impractical where cost considerations are important, e.g., for in-line, short transmission length applications such as in Local Area Networks (LANs).
A more cost-effective approach to filtering is to use a Fabry-Perot filter. Generally, a Fabry-Perot filter includes at least one pair of reflective elements, e.g. mirrors, separated by a fixed distance. By adjusting the distance between the reflective elements, the filter can be tuned to filter a selected channel. Advantageously, Fabry-Perot filters are less expensive and generate less optical loss than diffraction gratings.
One disadvantage associated with conventional Fabry-Perot filters is that they provide a very narrow resonant frequency passband, i.e. on the order of about 1-2% of the filter free spectral range (FSR). The narrow passband requires precise tuning of the filter to the signal transmitting and receiving elements, resulting in increased equipment costs. Moreover, where the filter is to be applied for filtering a plurality of spaced channels, i.e., as a comb filter, it is necessary to manufacture the filter with highly precise dimensions to ensure that the filter resonance frequencies match the desired transmission characteristics within the narrow passband. Accordingly, where Fabry-Perot filters have been used to filter a plurality of spaced channels, a separate filter has been used to separate each desired channel from the WDM signal. Using multiple Fabry-Perot filters having narrow frequency passbands is inefficient and expensive.
Accordingly, there is a need in the art for a Fabry-Perot filter which has an increased frequency passband compared to prior art designs, and which is capable of transmitting a first set of wavelengths from an input signal composed of a plurality of multiplexed optical signals and reflecting a corresponding second set of wavelengths. There is also a need in the art for a Fabry-Perot filter which may be efficiently and cost-effectively produced, and which may be used for transmitting a first set of wavelengths and reflecting a second set of wavelengths from an input composed of a plurality of multiplexed optical signals.